It is known for transformers, in particular integrated coreless transformers, to be used to transmit signals between DC-decoupled circuits. Transformers such as these are described, for example, in DE 101 00 282 A1 or DE 102 32 642. A circuit arrangement having a transformer as the signal transmission element, a transmission circuit which is connected on the primary side to the transformer and a receiver circuit which is connected on the secondary side to the transformer is described, for example, in DE 102 44 186 A1.
Owing to the lack of a ferromagnetic core, owing to the normally very small physical form in conjunction, and the use of thin metallization layers to produce the windings, transformers such as these are able to transmit only very short pulses or signals at very high carrying frequencies. The decay time constant (L/R) of a voltage pulse which can be tapped off on the secondary winding when a voltage pulse is applied to the primary winding is in the region of a few nanoseconds in the case of conventional coreless transformers with a diameter of about 400 μm, and the lower cut-off frequency (R/2πL) of a transformer such as this is accordingly more than 10 MHz. To a first approximation, this cut-off frequency is inversely proportional to the diameter of the planar windings of transformer, and increases further in the course of further miniaturization of the integrated transformers.
The receiver circuit, which is connected on the secondary side to the transformer, must be able to reliably detect the very short voltage pulses which are produced on the secondary side. This receiver circuit must have an upper cut-off frequency which is considerably above the lower cut-off frequency of the transformer.
It is known from the publication DE 102 44 186 A1 cited above as well as the publications WO 2002086969 A1 and EP 0 714 131 B1 for receiver circuits in signal transmission apparatuses to be produced with a coreless transformer using logic gates or Schmitt triggers using CMOS technology. The channel lengths of CMOS transistors must in this case be very short, in order to achieve short signal delay times in the receiver circuit, and thus a high upper cut-off frequency. The known receiver circuits, which are designed using CMOS technology, detect the voltage produced on the secondary coil and require signal levels on the secondary coil which are in the same order of magnitude as half the supply voltage of the receiver circuit. Only signals on the secondary side which are above this level are reliably identified as signal pulses.
However, CMOS transistors with short channel lengths can be produced only inaccurately, so that the switching thresholds and detector thresholds of the logic components which define the level of the signal pulses which can be processed may be subject to considerable manufacturing-dependent fluctuations. The receiver circuits must therefore be designed such that an adequate level margin is provided in order to make it possible to reliably evaluate a useful signal which is produced on the secondary coil.
Known receiver circuits furthermore have an input capacitance which is not negligible and which, together with unavoidable stray inductance on the secondary side of the transformer, forms a second-order low-pass filter, which limits the signal bandwidth of the signal pulses to be transmitted.
In the known receiver circuits, which tap the voltage off on the secondary winding, any stray capacitance on the secondary side of the transformer, which is normally present with respect to a substrate that is at the reference ground potential, also has a limiting effect on the signal bandwidth of processable signal pulses. It is known from WO 2002086969 A1, as cited above, and U.S. Pat. No. 6,420,952 B1, for a metallic shield to be provided between the primary winding and the secondary winding, and to be arranged closer to the secondary winding than to the primary winding. This metallic shield additionally increases this secondary stray capacitance.
In more recent receiver circuits produced using CMOS technology which can be finely structured, very thin gate oxides are used, which can easily be destroyed by over-voltages, caused by electromagnetic interference. Receiver circuits such as these must be protected by means of a surge arrestor, for example a zener diode. The depletion layer capacitance of the surge arrestor is connected in parallel with the input of the receiver circuit, and likewise has a bandwidth-limiting effect.
There is a need, therefore, for a signal transmission arrangement having a transformer and a receiver circuit, in which the receiver circuit reliably identifies the transmitted useful signal even in the case of a transformer with a short decay time constant, without having some or many of the disadvantages explained above.